Types of seal
- Axial seal: works along the axis of a shaft and prevents leakage by being compressed lengthwise between components. Used where there is no relative movement between the components being sealed.
- Radial seal: acts perpendicular to the shaft and retains fluids on rotating or reciprocating shafts, for instance in pumps and motors.
- O-ring: annular seal for static and dynamic applications. Withstands high pressure when correctly seated in its groove.
- Shaft seal: radially acting seal specifically for rotating shafts, preventing fluid escaping along the shaft.
- Lip seal: has one or more flexible lips pressed against the sealing surface. Typical of dynamic applications.
Materials
- NBR (nitrile rubber): widely used elastomer with very good resistance to oils, fuels and many chemicals. Working range roughly −40 to +100 °C.
- HNBR (hydrogenated nitrile rubber): hydrogenated variant of NBR with greater resistance to heat, ozone and chemical degradation. Working range roughly −40 to +150 °C.
- EPDM (ethylene propylene diene rubber): excellent resistance to weathering, ozone and water. Not resistant to mineral oil products.
- VMQ (silicone rubber): very flexible at low temperatures, thermally stable at high ones. Common in food, pharmaceutical and medical seals.
- FFKM (perfluoroelastomer): high-performance material with excellent resistance to heat, chemicals and solvents.
- PTFE (polytetrafluoroethylene): not an elastomer but a thermoplastic with very low friction and high chemical resistance.
- FKM / FPM (fluoroelastomer): known under the trade name Viton. Exceptional resistance to heat, oils, fuels and chemicals.
Characteristics and performance
- Compression set: the proportion of deformation that remains after the load is removed. A low value is desirable for lasting sealing performance.
- Abrasion resistance: resistance to wear through friction. Decisive for dynamic applications.
- Modulus of elasticity: a measure of stiffness — how much a material deforms under load.
- Gas permeability: how readily gases pass through the material. Where gas leakage is critical, FKM or PTFE are preferred.
- Chemical resistance: resistance to degradation by aggressive media.
- Resilience: the ability to return to the original shape after compression.
Installation and application
- Breakaway torque: the torque needed to initiate movement in the sealed system. Determined by the friction between seal and mating surface.
- Surface roughness (Ra): a measure of the texture of a surface. Smoother surfaces generally give better sealing performance.
- Clearance: the free space between seal and mating surface. Adequate clearance avoids excessive wear.
- Seal cross-section: the profile of the seal body, which helps determine the sealing force applied.
- Installation space: the space available for the seal. Size and type have to suit it.
Food industry regulations
- FDA 21 CFR 177.2600: regulation of the US FDA setting out which elastomers are permitted in food contact, including extraction tests.
- NSF 51: standard for safety and hygiene requirements for materials in commercial food equipment.
- BfR XXI, category 4: German standard of the Federal Institute for Risk Assessment for rubber and silicone materials in food contact.
- EU 1935/2004: European framework regulation for all food contact materials.
- GMP (good manufacturing practice): system ensuring that products are consistently produced and tested to defined quality standards.
In the original post the last entry broke off mid-sentence and was completed during the migration. The material data of our qualities are given in the elastomer overview.